Now I can also confirm that neither $HistoryLength = 0 nor
ClearSystemCache[] has any effect on the memory consumption of
Mathematica when using ListContourPlots and similar functions. The
memory usage, indicated by the "top" command (Linux), is still large.
More annoying is the fact that this slows down the Mathematica
interface enormously: clicking on some plot now takes a few seconds
until the plot is selected, saving your notebook can take up to 30
seconds. Maybe this is also related to the general "strangeness" of
the Linux version of the notebook interface.
My system: openSuse 10.3